The goal of our study was to compare the morphology, survival and frequency of reproduction of Smooth snakes (Coronella austriaca) between The Netherlands and Norway. Individuals caught in the field were measured and identified using photographs. We here archive the raw measurements and individual encounter histories used in the analyses of Dalessi et al. (2021, Amphibia-Reptilia).

Field observations were done in the Eindhoven area in The Netherlands (51°27′0″N, 5°28′0″E), and the Oslo area (59°54′41″N, 10°45′29″E) in Norway. In The Netherlands the study area consists of a number of remaining nature fragments of De Peel, a once large moorland area in the South-east of The Netherlands. Fragmentation is largely due to peat extraction and transformation of the former peat bog into farmland. De Peel is located across and along the border of the provinces Noord-Brabant and Limburg. Two large and a number of smaller geographical elements can be distinguished that still show most of the original natural conditions and features of a peat-bog moor. De Groote Peel (1500 ha; a national park) is one of these, the other is a combined area consisting of the Deurnsche Peel (1400 ha) and the adjacent Mariapeel (1400 ha). The vegetation in these sites is dominated by Purple moor-grass (Molinea caerulea). Other plants species that are mainly found are common heather (Calluna vulgaris), cross-leaved heath (Erica tetralix), small trees and different species of peat moss (Sphagnum spp.). In this study we included six locations within these two nature reserves: Groote Peel Yellow track, Groote Peel summer biotope, Groote Peel Noordoostpad, Mariapeel East, and in the Deurnsche Peel: Leegveld and Eikenlaan. In the South of Norway, Coronella austriaca is found mainly in a narrow area along the southwestern coastline. Typical habitat is more or less isolated open areas (surrounded by forest varying in density) with south-facing rocky slopes. Patches of small trees and shrubs are present in these areas and particularly crevices and parts where cracks filled with plant material, loose rocks and stones occur, are the spots where smooth snakes were found. Typical plant species are Scotch pine (Pinus silvestris), juniper (Juniperus communis) and birch (Betula verrucosa). Heather (Calluna vulgaris) is dominating the lower vegetation (Sørensen 2014). This study includes data from 8 different locations (mainly fjord-related habitats) south of Oslo (Sørensen 2014): Pollevannet, Skjelvik, Emmerstad, Bunnefjorden, Eineåsen, Digerud, Tofte and Bleikslitjern.

In The Netherlands we searched for smooth snakes during their reproductive season (April/May to August/September) from 2011 until 2015. Visiting frequencies were higher from 2012 onwards (ranging from approximately 40 visits in 2011, 60 visits in 2012, 80 visits in 2013 to around 130 visits during 2014 and 150 visits during 2015, for at least once a week in all years), and were higher in Leegveld and Mariapeel East than in the other areas. Observations in Norway were collected with frequencies varying from 1 to 20 visits per year varying with weather conditions and available time. In The Netherlands, we constructed encounter histories for a total of 110 distinguished individual female snakes. A number of these individuals were observed during multiple years, adding up to a total of 157 observations, 142 of which involved pregnant females and 9 observations non-pregnant females. In 6 cases the pregnancy status was not established with certainty. Females were determined to be pregnant or non-pregnant visually (in many cases the unborn snakes can readily be observed) or by means of palpating in both Norway and The Netherlands (Reading 2004). A number of pregnant females was temporarily held in captivity to determine the number of offspring. Physical traits such as sex, total body length (summation of the snout-vent length and tail length; measured with measuring tape) were recorded. Body mass was determined by use of a scale-beam (Super Samson, 200 g max., increments 2 g). Temperature was recorded as well. Photographs of the heads of all snakes (from above) and first 5-15 cm of dorsal side of the body were taken in order to be able to distinguish individual snakes, based on coloration and markings present on the skin (Sauer, 1994). Each individual was marked with a small dot of green nail polish on the head and thereafter released at the exact same spot where it was captured. In Norway, measurements of participation in reproduction of female snakes were carried out during a varying number of consecutive years between 1982 and 2015. Generally, more observations were made per individual, but for fewer individuals than in The Netherlands. Exact locations of the found individuals were determined and measurements such as total body length, body mass and whether females were pregnant or not were taken using the same methods as in The Netherlands. Females were photographed in order to distinguish between individuals. Encounter histories were constructed for a total of 87 distinguished individual female snakes from Norway. A number of these snakes were observed during multiple years, adding up to a total of 184 observations, 148 of which were determined pregnant, and 29 observations were determined non-pregnant. In 7 cases pregnancy status was not established with certainty.